a) Field of the Invention
This invention relates to a hydrodynamic bearing apparatus which relatively rotatably supports a shaft and a bearing via hydrodynamic pressure generated by a lubricant filled in a bearing clearance.
b) Description of the Related Art
Conventionally, various types of hydrodynamic bearing apparatus are known which relatively rotatably support a shaft and a bearing via hydrodynamic pressure generated by a lubricant for the bearing For example, in the hydrodynamic bearing apparatus used for a spindle motor for a driving disk, and the like, a lubricant (lubricant fluid) consisting of oil and the like is filled between the facing hydrodynamic surfaces of a metal, which is made of copper or copper alloy and used as a radial bearing, and a shaft made of alloy steel and the like, such as stainless steel containing chrome, to form a noncontact abrasive section with the hydrodynamic pressure of the lubricant fluid.
In the hydrodynamic bearing apparatus mentioned above, the shaft contacts the bearing at the initial stage of rotation, and therefore, the material wears down due to "galling" and "burning." To reduce the level of wearing-out, a relatively soft material such as copper or copper alloy is used to construct the bearing. In addition to using copper or copper alloy for a bearing, using an alloy steel such as stainless steel for a shaft, the level of wearing-out can be reduced; and because copper or copper alloy is easy to machine in general, the hydrodynamic pressure generating grooves and the like on the inner circle surface of the bearing can be machined easily. Nevertheless, the difference in thermal expansion coefficient for the bearing and the shaft in the above mentioned configuration varies the so-called bearing clearance therebetween according to the change in environmental temperature; it also varies the viscosity of the lubricant fluid, resulting in fluctuation of bearing properties.
To resolve the problems such as the bearing clearance variation due to the changes in environmental temperature and the viscosity fluctuation in the lubricant fluid, a steel material such as stainless steel (SUS) or free-machining steel (SUM24L) whose linear expansion coefficient is small, is used for a bearing so that the excellent bearing properties can be kept regardless of the changes in environmental temperature.
The above mentioned hydrodynamic bearing apparatus, however, uses a steel material having a low expandability and a small linear expansion coefficient; therefore, it causes severe wearing-out due to galling and burning. Also, since the material is not easy to machine, it is difficult to machine precisely the hydrodynamic pressure generating grooves and the like into the inner surface of the bearing. Especially obtaining a precise surface roughness, flatness, and the like is difficult. In the worst case, even the bearing properties are deteriorated.